codec C. Hoene, Ed.
Internet-Draft Universitaet Tuebingen
Intended status: Informational JM. Valin
Expires: April 26, 2012 Mozilla Corporation
K. Vos
Skype Technologies S.A.
J. Skoglund
Google
October 24, 2011
Summary of Opus listening test resultsdraft-ietf-codec-results-00
Abstract
This document describes and examines listening test results obtained
for the Opus codec and how they relate to the requirements.
Status of this Memo
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Internet-Draft Codec Quality October 20111. Introduction
This document describes and examines listening test results obtained
for the Opus codec. Some of the test results presented are based on
older versions of the codec or on older versions of the SILK or CELT
components. While they do not necessarily represent the exact
quality of the current version, they are nonetheless useful for
validating the technology used and as an indication of a lower bound
on quality (based on the assumption that the codec has been improved
since they were performed).
Throughout this document, all statements about one codec being better
than or worse than another codec are based on 95% confidence. When
no statistically significant difference can be shown with 95%
confidence, then two codecs are said to be "tied".
In addition to the results summarized in this draft, Opus has been
subjected to many informal subjective listening tests, as well as
objective testing.
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Internet-Draft Codec Quality October 20112. Opus listening tests on final bit-stream
The following tests were performed on the Opus codec _after_ the bit-
stream was finalized.
2.1. Google listening tests
The tests followed the MUSHRA test methodology. Two anchors were
used, one lowpass-filtered at 3.5 kHz and one lowpass-filtered at 7.0
kHz. Both trained and untrained listeners participated in the tests.
The reference signals were manually normalized to the same subjective
levels according to the experimenters' opinion. Experiments with
automatic normalization with respect to both level and loudness (in
Adobe Audition) did not result in signals having equal subjective
loudness. The sample magnitude levels were kept lower than 2^14 to
provide headroom for possible amplification through the codecs.
However, the normalization exercise was not repeated with the
processed sequences as neither the experimenters nor any of the
subjects (which included expert listeners) noticed any significant
level differences between the conditions in the tests. The only
post-processing performed was to remove noticeable delays in the MP3
files, as one could identify the MP3 samples when switching between
conditions when the MP3 had the longer delay. The testing tool Step
from ARL was used for tests and all listeners were instructed to to
carefully listen through the conditions before starting the grading.
The results of the tests are a available on the testing slides
presented at the Prague meeting [Prague-80].
2.1.1. Google narrowband listening test
The test sequences in Test 1 were mono recordings (between 2 and 6
seconds long) of 4 different male and 4 different female speakers
sampled at 48 kHz in low background noise. 17 listeners were
presented with 6 stimuli according to Table 1 for each test sequence.
The corresponding bit rate for the reference is 48000 (sampling
frequency in Hz) x 16 (bits/sample) = 768 kbps. Since the anchors
are low-pass filtered they can also be downsampled for transmission
which corresponds to lower bit rates. Three narrowband codecs were
compared in this test: Opus NB, the royalty-free iLBC, and the
royalty-free Speex. The codecs all have an encoder frame length of
20 ms. Both Opus and Speex had variable rate whereas iLBC operated
at a fixed bit rate.
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The results indicate that all codecs had comparable performance,
except for G.719, which had a considerably lower score. T-tests by
Greg Maxwell verified that the low-delay Opus at 128 kbps had a
significantly higher performance and that G.719 had a significantly
lower performance than the other four.
2.1.4. Google transcoding test
If two telephone networks of different technology are coupled,
frequently speech has to be transcoded: It must be decoded and
encoded before it can be forward to the next network. Then, two
codecs are cooperating in a row, which is called tandem coding.
In the following tests, Jan Skoglund studied the impact of
transcoding if Opus call is forwarded to a cellular phone system.
[Skoglund2011]. Two tests were conducted for both narrowband and
wideband speech items. The test conditions of the narrow-band tests
are given in Table and the respective results in . For the wide-band
conditions and results refer to Table and .
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+------------+--------------------------+--------+
| Test Item | Subjective BS.1587 Score | 95% CI |
+------------+--------------------------+--------+
| Reference | 98.95 | 0.59 |
| | | |
| Opus 32 | 98.13 | 0.72 |
| | | |
| G.719 32 | 93.43 | 1.51 |
| | | |
| Opus 20 | 81.59 | 2.48 |
| | | |
| LP 7 | 79.51 | 2.53 |
| | | |
| G.722.1 24 | 72.55 | 3.06 |
| | | |
| LP 3.5 | 54.57 | 3.44 |
| | | |
| Speex 24 | 53.63 | 4.23 |
+------------+--------------------------+--------+
Table 11: Mandarin wideband speech: test results
Under the given confidence intervals, the quality of Opus at 11 kbps
equals the quality of iLBC at 15 kbps and the quality aferlowpass
filtering at 3.5 kHz. Speex at 11 kbps does not perform as well.
According to the listening-only tests, Opus at 32 kbps is better than
G.719 at 32 kbps. Opus at 20 kbps outperforms G.722.1 and Speex at
24 kbps. If one compares the Mandarin results with those for English
(Section 2.1.1 and Section 2.1.2), one can see that are pretty
consistent. The only difference is that using English stimuli Opus
at 20 kbps outperforms G.719 at 32 kbps. Probabily, this is due to
the fact that Mandarin speech does not contain as many high
frequency-rich consonants such as [s] as English.
2.2. HydrogenAudio stereo music listening test
In March 2011, the HydrogenAudio community conducted a listening test
comparing codec performance on stereo audio at 64 kb/s [ha-test].
The Opus codec was compared to the Apple and Nero implementations of
HE-AAC, as well as to the Vorbis codec. The test included 30 audio
samples, including known "hard to code" samples from previous
HydrogenAudio listening tests.
A total of 33 listeners participated in the test, 10 of which
provided results for all the audio samples. The results of test
showed that Opus out-performed both HE-AAC implementations as well as
Vorbis.
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Internet-Draft Codec Quality October 20112.3. Nokia Interspeech 2011 listening test
In 2011, Anssi Ramo from Nokia submitted [Ramo2011] the results of a
second listening test, focusing specifically on the Opus codec, to
Interspeech 2011. As in the previous test, the methodology used was
a 9-scale ACR MOS test with clean and noisy speech samples.
The results show Opus clearly out-performing both G.722.1C and G.719
on clean speech at 24 kb/s and above, while on noisy speech all
codecs and bit-rates above 24 kb/s are very close. It is also found
that the Opus hybrid mode at 28 kb/s has quality that is very close
to the recent G.718B standard at the same rate. At 20 kb/s, the Opus
wideband mode also out-performs AMR-WB, while the situation is
reversed for 12 kb/s and below. The only narrowband rate tested is 6
kb/s, which is below what Opus targets and unsurprisingly shows
poorer quality than AMR-NB at 5.9 kb/s.M
2.4. Universitaet Tuebingen stereo and binaural tests
Modern teleconferencing system use stereo or spatialy rendered speech
to enhance the conversation quality. Then, talkers can be identified
according to their acoustic locations. Opus allows to encode speech
in a stereo mode. In the tests conducted by Christian
Hoene[Hoene2011], the performance of Opus coding stereo and binaural
speech was studied.
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Internet-Draft Codec Quality October 20113. Conclusion on the requirements
The requirements call for the Opus codec to be better than Speex and
iLBC in narrowband mode, better than Speex and G.722.1 in wideband
mode, and better than G.722.1C in super-wideband/fullband mode.
3.1. Comparison to Speex (narrowband)
The Opus codec was compared to Speex in narrowband mode in the Google
narrowband test (Section 2.1.1). This test showed that Opus at 11
kb/s was significantly better than Speex at the same rate. In fact,
Opus at 11 kb/s was tied with the 3.5 low-pass of the original.
Considering the results, we conclude that the Opus codec is better
than the Speex codec.
3.2. Comparison to iLBC
The Opus codec was compared to iLBC in the Google narrowband test
(Section 2.1.1). This test showed that Opus at 11 kb/s was
significantly better than iLBC running at 15 kb/s. Considering the
results, we conclude that the Opus codec is better than the iLBC
codec.
3.3. Comparison to Speex (wideband)
The Opus codec was compared to Speex in wideband mode in the Google
wideband and fullband test (Section 2.1.2). This test showed that
Opus at 20 kb/s was significantly better than Speex at at 24 kb/s.
In fact, Opus at 20 kb/s was better than the 7 kHz low-pass of the
original. These results are consistent with an earlier Dynastat test
(Appendix A.1) that also concluded that SILK had significantly higher
quality than Speex in wideband mode at the same bit-rate.
Considering the results, we conclude that the Opus codec is better
than the Speex codec for wideband.
3.4. Comparison to G.722.1
In the Google wideband and fullband test (Section 2.1.2), Opus at 20
kb/s was shown to significantly out-perform G.722.1 operating at 24
kb/s. An indirect comparison point also comes from the Nokia
Interspeech 2011 listening test (Section 2.3) that shows Opus out-
performing AMR-WB at 20 kb/s, while AMR-WB is known to out-perform
G.722.1. Considering these results, we conclude that the Opus codec
is better than the G.722.1 codec for wideband.
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Internet-Draft Codec Quality October 20113.5. Comparison to G.722.1C
Opus has been compared to G.722.1C in multiple listening tests. As
early as 2008, an old version of the CELT codec (Appendix A.4) using
very short frames was found to have higher quality than G.722.1C at
48 kb/s. More recently, the Nokia Interspeech 2011 listening test
(Section 2.3) showed that Opus out-performed G.722.1C at 24 kb/s, 32
kb/s, and 48 kb/s. We thus conclude that the Opus codec is better
than the G.722.1C codec for superwideband/fullband audio.
3.6. Comparison to AMR-NB
In the Google narrowband test (Section 2.1.1), Opus was shown to out-
perform AMR-NB at 12 kb/s. On the other hand, in the Nokia
Interspeech 2011 listening test (Section 2.3), AMB-NB was found to
have better quality than Opus at 6 kb/s. This indicates that Opus is
better than AMR-NB at higher rates and worse at lower rates, which is
to be expected given Opus' emphasis on higher quality and higher
rates.
3.7. Comparison to AMR-WB
In the Google wideband and fullband test (Section 2.1.2), Opus at 20
kb/s was shown to out-perform AMR-WB at the same rate. This was also
confirmed by the Nokia Interspeech 2011 listening test (Section 2.3),
with also found AMR-WB to out-perform Opus at 12 kb/s and below. As
with AMR-NB, we conclude that Opus is better than AMR-WB at higher
rates and worse at lower rates.
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Internet-Draft Codec Quality October 20116. Acknowledgments
The authors would like to thank Anssi Ramo and the HydrogenAudio
community, who conducted some of the Opus listening test cited in
this draft.
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Internet-Draft Codec Quality October 2011Appendix A. Pre-Opus listening tests
Several listening tests have been performed on the SILK and CELT
codecs prior to them being merged as part of the Opus codec.
A.1. SILK Dynastat listening test
The original (pre-Opus) SILK codec was characterized in a Dynastat
listening test [SILK-Dynastat]. The test included 32 conditions with
4 male and 4 female talkers. The test signals were wideband speech
with and without office background noise at 15 dB SNR. Packet loss
was tested at 2, 5, and 10% loss rates. The bitrates ranged from
8.85 kb/s to 64 kb/s. The codecs included in the test were SILK-WB,
AMR-WB, Speex-WB and G.722 (which ran at 64 kb/s).
The results showed that for clean speech (1) SILK out-performs AMR-WB
at all bit-rates except 8.85 kb/s (which was a tie); (2) SILK out-
performs Speex at all bit-rates; and (3) SILK running at 18.25 kb/s
and above out-performs G.722 at 64 kbps. For noisy speech, tested at
18.25 kb/s, SILK is tied with AMR-WB, and out-performs Speex. For 2,
5 and 10% packet loss, tested at 18.25 kb/s, SILK out-performs both
AMR-WB and Speex in all conditions.
A.2. SILK Deutsche Telekom test
In 2010 Deutsche Telekom published results [Wustenhagen2010] of their
evaulation of super-wideband speech and audio codecs. The test
included the version of SILK submitted to the IETF. The results
showed that for clean speech (item "speechsample") SILK was tied with
AMR-WB and G.718, and out-performed Speex. For noisy speech (item
"arbeit") SILK out-performed AMR-WB and G.718 at 12 and 24 kb/s, and
Speex at all bitrates. At bitrates above 24 kb/s SILK and G.718 were
tied.
A.3. SILK Nokia test
In 2010, Anssi Ramo from Nokia presented [Ramo2010] the results of a
listening test focusing on open-source codecs at Interspeech 2010.
The methodology used was a 9-scale ACR MOS test with clean and noisy
speech samples.
It was noted in the test that:
"Especially at around 16 kbit/s or above Silk is better than AMR-WB
at comparable bitrates. This is due to the fact that Silk wideband
is critically sampled up to 8 kHz instead of ITU- T or 3GPP defined 7
kHz. This added bandwidth (from 7 to 8 kHz) shows up in the results
favourable to Silk. It seems that Silk provides quite artifact free
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voice quality for the whole 16- 24 kbit/s range with WB signals. At
32 and 40 kbit/s Silk is SWB and competes quite equally against
G.718B or G.722.1C although having a slightly narrower bandwidth than
the ITU-T standardized codecs."
A.4. CELT 0.3.2 listening test
The first listening tests conducted on CELT version 0.3.2 in 2009 and
published in 2010 [valin2010] included AAC-LD (Apple), G.722.1C and
MP3 (Lame). Two MUSHRA tests were conducted: a 48 kb/s test and a 64
kb/s test, both at a 44.1 kHz sampling rate. CELT was used with 256-
sample frames (5.8 ms). All codecs used constant bit-rate (CBR).
The algorithmic delay was 8.7 ms for CELT, 34.8 ms for AAC-LD, 40 ms
for G.722.1C and more than 100 ms for MP3.
The 48 kb/s test included two clean speech samples (one male, one
female) from the EBU SQAM database, four clean speech files (two
male, two female) from the NTT multi-lingual speech database for
telephonometry, and two music samples. In this test, CELT out-
performed AAC-LD, G.722.1C and MP3.
The 64 kb/s test included two clean speech samples (one male, one
female) from the EBU SQAM database, and six music files. In this
test, AAC-LD out-performed CELT, but CELT out-performed both MP3 and
G.722.1C (running at its highest rate of 48 kb/s).
A.5. CELT 0.5.0 listening test
Another CELT listening test was conducted in 2009 on version 0.5.0
and presented at EUSIPCO 2009 [valin2009]. In that test, CELT was
compared to G.722.1C and to the Fraunhofer Ultra Low-Delay (ULD)
codec on 9 audio samples: 2 clean speech samples and 7 music samples.
At 64 kb/s with 5.3 ms frames, CELT clearly out-performed G.722.1C
running at 48 kb/s with 20 ms frames. Also, at 96 kb/s and equal
frame size (2.7 ms), CELT clearly out-performed the ULD codec.
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Internet-Draft Codec Quality October 2011Appendix B. Opus listening tests on non-final bit-stream
The following listening tests were conducted on the Opus codec on
versions prior to the bit-stream freeze. While Opus has evolved
since these tests were conducted, the results should be considered as
a _lower bound_ on the quality of the final codec.
B.1. First hybrid mode test
In July 2010, the Opus codec authors conducted a preliminary MUSHRA
listening test to evaluate the quality of the recently created
"hybrid" mode combining the SILK and CELT codecs. That test was
conducted at 32 kb/s and compared the following codecs:
o Opus hybrid mode (fullband)
o G.719 (fullband)
o CELT (fullband)
o SILK (wideband)
o BroadVoice32 (wideband)
The test material consisted of two English speech samples from the
EBU SQAM (one male, one female) database and six speech samples
(three male, three female) from the NTT multi-lingual speech database
for telephonometry. Although only eight listeners participated to
the test, the difference between the Opus hybrid mode and all other
codecs was large enough to obtain 95% confidence that the Opus hybrid
mode provided better quality than all other codecs tested. This test
is of interest because it shows that the hybrid clearly out-performs
the codecs that it combines (SILK and CELT). It also out-performs
G.719, which is the only fullband interactive codec standardized by
the ITU-T. These results were presented [Maastricht-78] at the 78th
IETF meeting Maastricht.
B.2. Broadcom stereo music test
In December 2010, Broadcom conducted an ITU-R BS.1116-style
subjective listening test comparing different configurations of the
CELT-only mode of the IETF Opus codec along with MP3 and AAC-LC. The
test included stereo 10 audio samples sampled at 44.1 kHz and
distributed as follows:
o 2 pure speech
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o 2 vocal
o 2 solo instruments
o 1 rock-and-roll
o 1 pop
o 1 classical orchestra
o 1 jazz
A total of 17 listeners participated to the test. The results of the
test are a available on the testing slides presented at the Prague
meeting [Prague-80]. Although at the time, Opus was not properly
optimised for 44.1 kHz audio, the quality of the Opus codec at 96
kb/s with 22 ms frame was significantly better than MP3 and only
slightly worse than AAC-LC. Even in ultra low-delay mode (5.4 ms),
Opus still outperformed MP3. The test also confirmed the usefulness
of the prefilter/postfilter contribution by Raymond Chen, showing
that this contribution significantly improves quality for small
frames (long frames were not tested with the prefilter/postfilter
disabled).
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Internet-Draft Codec Quality October 2011Appendix C. In-the-field testing
Various versions of Opus (or SILK/CELT components) are currently in
use in production in the following applications:
o Skype: VoIP client used by hundreds of millions of people
o Steam: Gaming distribution and communications platform with over
30 million users
o Mumble: Gaming VoIP client with more than 200 thousand users
o Soundjack: Client for live network music performances
o Freeswitch: Open-source telephony platform
o Ekiga: Open-source VoIP client
o CHNC: Radio station using CELT for its studio-transmitter link
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